fix for latest flake8

tensorpack/utils/concurrency.py

@@ -231,10 +231,11 @@ def start_proc_mask_signal(proc):
         for p in proc:
             if isinstance(p, mp.Process):
                 if sys.version_info < (3, 4) or mp.get_start_method() == 'fork':
-                    log_once(
-"Starting a process with 'fork' method is not safe and may consume unnecessary extra CPU memory."
-" Use 'forkserver' or 'spawn' method (available after Py3.4) instead if you run into any issues. "
-"See https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods on how to set them.",
+                    log_once("""
+Starting a process with 'fork' method is not safe and may consume unnecessary extra CPU memory.
+Use 'forkserver' or 'spawn' method (available after Py3.4) instead if you run into any issues.
+See https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods on how to set them.
+""".replace("\n", ""),
 'warn')  # noqa
             p.start()